Alternating current motor control



March 14, 1950 w, JACQBSON 2,500,314

ALTERNATING CURRENT ue'ron comm.

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Patented Mar. 14, 1950 ALTERNATING CURRENT MOTOR CONTROL Abraham Walter Jacobson, New Haven, Coma. assignor to The Bristol Company, Waterbury, Coma, a corporation of Connecticut Application August 10. 1944, Serial No. 548,820

2 Claims.

This invention relates to electric motor control systems and circuits, and more especially to the control of small alternating-current motors and of servomotors in such apparatus as automatic regulators, steering mechanisms and selfbalancing networks applying null principles in the measurement of electrical magnitudes. In the majority of applications of servomotors it is desired that the performance of the balancing mechanisms, or whatever apparatus is being adjusted by said servomotor, be as nearly as possible critical in its characteristic. That is, the ideal performance would involve the adjusted elements responding instantaneously to any change in the commanding magnitude. and coming to rest abruptly at the newly determined balance point or condition. As the absolute attainment of this ideal is, of course, rendered impossible due to the property of inertia possessed by all bodies, it has been customary to approximate the desired condition by providing the motor or associated mechanism with some form of braking device, either mechanical or electrical, "which, applying a suitable counter-influence as a balance condition is approached, tends to offset the overshooting eflect of inertia, and more or less abruptly bring the mechanism to. rest as the balance point is attained.

It is an object of the present invention to provide electrical means for damping or braking a servomotor as a balance point is approached.

It is a further object to provide electrical braking means which shall be wholly external to the controlled motor, and therefore readily adaptable to a conventional form of motor without electrical or structural modification of the same.

It is a further object to provide braking means or the above nature, in which the retarding influence upon the moving part of the motor shall be at a maximum as the balance point is approached and shall be withdrawn to a greater or less degree as the elements of the balancing system depart from a balanced condition.

It is a further object to provide a braking system of the above nature which shall be effective without respect to the direction of rotation of the motor to which it is applied.

Other features and advantages of the invention will be hereinafter described and claimed.

In carrying out the purposes of the invention in the embodiments here illustrated, it is proposed to provide a servomotor of the class having two geometrically displaced windings, one continuously energized from a supply of alternating current having a fixed frequency, and the other with alternating current of the same frequency. but of an intensity and phase-position depend ing upon the extent and direction of departure of the controlled system from a balance condition. and to superimpose upon said last-named current supplementary current having a frequency dill'ering to a material degree from that of said supply. In such an arrangement, the electromagnetic forces derived from the interaction of said currents of supply frequency will be such as to cause said motor to operate with an intensity and in a direction related to the magnitude of the variable current and its phase positlon with respect to that of the constant alternating excitation; and, while the reaction between the supplementary current upon the magnetic field derived from the supply frequency will not be such as to produce any tendency for continuous rotation of the motor, the field produced by said current will react with the moving conductors of the motor to produce a retarding effect substantially independent of the frequency of said current.

Because of the relative ease with which whole multiples of a given frequency of alternating current may be obtained from said frequency by the use of non-linear networks or other devices involving no moving parts, it is in general preferable that the supplementar current utilized to provide a retarding effect on the motor under control have a frequency so related to that of the supply; and both theoretical computations and experimental data demonstrate that eminently satisfactory results may be obtained by the use of a frequency double that of the source of supply.

In the drawings:

Figs. 1 and 2 are diagrammatic representations of simple motor controls embodying the principle of the invention.

Fig. 3 is a diagram of a reversible motor control embodying the principle of the invention.

Fig. 4 is a diagram of an electronic control circult whereby the principle of the invention may be expediently utilized in a self-balancing bridge network.

Fig. 5 is a diagram of a ferromagnetic induction modulator inherently embodying the principle of the invention.

Fig. 6 is a diagram of the device shown in Fig. 5 applied to the recording of thermoelectrically determined temperature values.

Referring now to the drawings:

Fig. l is a diagram of a simple control circuit adapted to the operation of an induction motor by current derived from an A.-C. supply source having a specified frequency, and the braking of the same motor by a current of another frequency introduced in one of its windings. A motor II has wound upon its stator structure two primary windings I l and I2, geometrically displaced from one another, the former adapted to be continuously energized from a. suitable source it of alternating current having a fixed frequency and voltage, said frequency value being referred to as "N cycles per second, and the latter winding having terminals adapted for energize-tion from either or both of two sources presently to be described. The motor HI is provided with a rotor member It having thereon a short-circuited secondarywinding, whereby, according to the wellknown principle of the induction motor, torque may be developed in said rotor by the interaction of the alternating magnetic fields established by currents in the primary windings p n currents inducedinsaidsecondarywindingbysaidalternating fields. In order that astarting torque be developed in an induction motor it is essential that the field produced by the primary windings be progressive or rotating in its nature; and this demands that the two primary windings be not only geometrically displaced in the sense of rotation, but also energized by currents differing in time-phase. To this end, there is provided a. phase-shifting transformer Ii, or equivalent phase-shifting device, adapted to be energined from the A.-C. source It, and to produce an output voltage of the same frequency as that of the supply, but having a definite time-phase displacement therefrom. The output terminals of the phase-shifting transformer Ii are connected, in series with a suitable switch it (here shown as of the normally-open push-button type) on which the primary winding I! of the motor Hi may be energized and de-energized at will.

In the apparatus as thus far described there is provided a two-phase induction motor of a conventional type, in which energization of the primary winding H alone will not cause a starting torque to be developed, but in which, with closure of the switch 16, the rotating field produced by the combined action of the two primary windings will cause to be induced in the shortcircuited secondary winding of the rotor 14 such currents, as, reacting upon the stator field, will set up a torque and cause the rotor to revolve in a sense determined by the relative geometrical and phase displacements of the primary windings and their energizing currents.

Upon opening of the switch IS, the stator will remain energized by current in the winding II; and, according to the design and proportioning of the motor HI, the rotor will either continue to revolve in accordance with the principle of the single-phase induction motor, or lacking a. sufficient intensity of rotating or progressing field, will by its own inertia coast to a position of rest.

In order to obtain the desired retarding or braking action, there is provided for the field If a supplementary source of excitation ll. This source comprises a source of alternating current having a frequency differing materially from that of the supply l3; and the connection with the winding 12 is made such that currents from the sources l3 and I! may fiow simultaneously and without mutual interference through said winding. with connections as shown in Fig. 1, the winding 12 is continuously energized from the source Il, while, as hereinbefore pointed out, the winding II is continuously energized from the source l3. If the frequencies-of the two sources differed by but a small amount, e. g., one cycle per second, the resultant tendency would be for the rotor II to oscillate with a frequency representing that difference; but as the disparity between the respective frequencies is made great, the oscillatory tendency is absorbed by the inertia of the rotor, and either disappears entirely, or is apparent only in a slight vibratory effect. It has been found in practice that optimum results are obtained when the frequency of the auxiliary source I! bears to that of the supply source II a ratio which is a whole number (preferably even) which may be designated as "C," whence the frequency of the auxiliary source may be referred to as CN cycles per second. It is within the spirit of the invention that the value of C might be zero, in which case the supplementary source of excitation would be direct current.

When the switch I8 is closed, permitting alternating current of normal supply frequency to flow in the winding I! simultaneously with current from the source H, the normal frequency componentof the current flowing in said winding will react in the manner hereinbefore set forth with that fiowing in the winding H to cause rotation of the rotor I4. While the current from the source 11 will not appreciably affect the rotative tendency of the moving part, it will pro- 'vide an alternating field of corresponding frequency which will be cut by the conductors of the rotor winding, causing short-circuit or eddy currents to flow therein, and introducing a corresponding damping action upon the rotor without respect to frequency of the generating flux. If the intensity of the normal frequency current be made sufllclent to overcome this damping action, the rotor will operate, but will be subject to a permanently applied retarding influence which, to some extent, will tend to reduce the speed of the operation when current of the normal supply frequency is flowing in the wind ing l2. Upon opening of the switch "5, and removal of normal frequency energization from the winding II, the effect of eddy currents set up in the rotor due to the interaction of its winding with the flux produced by the supplementary current will quickly bring the rotor to rest. The magnitude of the supplementary current flowing in the winding 12 may be adjusted to such a value that, while its effect is not suflicient to interfere seriously with operation of the motor when subjected to two-phase energization, it will be sufilcient to overcome any tendency of the motor to operate under single phase excitation and will provide a positive braking eifect. The flux due to the continuous excitation of the primary winding Ii also interacts with the moving conductors of the rotor H to produce a certain retarding action; but this eifect is to such an extent involved with the forces tending to produce rotation as a single-phase motor that any attempt to utilize said flux for braking purposes seriously restricts the flexibility of design and operation of the apparatus, and is usually practicable within an exceedingly narrow range of operating conditions.

While it is quite practicable to utilize for braking purposes the retarding eflect of a supplementary current of other than normal operating frequency, continuously applied as in Fig. l, I provide in Fig. 2 for material improvement in the efficiency of operation through arrangements for applying the supplementary current to the aeocsu stator winding alternatively to the driving current, so that the two currents do not ilow in said winding simultaneously. The arrangement shown inl'ig.2isina1lrespectsidenticaltothatshown in PK. 1, with exception of the fact that the normallyopenswitch llshowninl'imiiareplaced by a double-throw switch II. Said switch B normally maintained in a position to apply current from the supplementary source I! to the primary winding I! of the motor, and adapted, when manually actuated, to interrupt connections between said source and said winding, and toplacethelatterincommunicationwiththe output terminals of the phase-shitting transformer II. It will be seen that, with the arrangement shown in Fig. 2, the braking influence of the current derived from the source I! will be present in the motor at all times excepting when the switch ll is thrown to the operating position. At that time the motor will operate freely as a two-phase motor without any retarding eilect due to supplementary current, and, as soon as the driving influence is removed, the supplementary current will flow in the stator winding, introducing the retarding eifect, whereby, as herelnbefore set forth the motor will quickly be brought to rest.

In Fig. 3 is shown a system whereby the principle of the invention may be applied to the control of a reversible motor in such a manner that the retarding eifect is present only at those times when the motor is not subjected to normal driving influences. A motor similar in all respects to the motor ill shown in Figs. 1 and 2, has wound upon its stator structure two mutually displaced primary windings 26 and 21 and is provided with a rotor member." having thereon a short-circuited secondary winding. The primary winding 26 is continuously energized from an alternating current source 28 of constant frequency and voltage. A phase-shifting transformer 30 has its primary terminals connected to the source 29,

and its secondary terminals to an inductive winding 3i having a mid-tap, whereby there may be obtained a potential midway between that of the output terminals of said phase-shifting transformer. In certain types of phaseshifting apparatus, it is practicable to include such a mid-tap as a part of the transforming or converting unit, in which case the tapped inductance 3| will not be necessary. .A threepoint switch 32 having end contacts 83 and SI and an intermediate contact 35 adapted to be alternatively engaged by a movable contact arm 38, has said end contacts It and it directly connected to the output terminals oi the phaseshiiting device 30. The terminals of the primary winding 21 of the motor 25 are connected respectively to the mid-tap of the inductance II and to the movable contact am It.

Also supplied from the source I! is a frequency converter 31 adapted to provide at its output terminals an alternating voltage of suitable magnitude, and of frequency difl'ering from that of the source. Frequency converters having no moving parts are commonly available, and need not here be described. A device for producing an output frequency three times that of the supply is described in the book Measurement of Inductance, Capacitance, and Frequency" by Campbell and Chllds (MacMillan, London, 1935) page 38. Devices whereby a double frequency may be derived from an A.-C. source are described in U. 8. Letters Patent No. 2,324,634, granted to H. J. McCreary July 20, 1943, and in Patent No.

0 2,849,057, granted to H. P. Eton-m, May 16, 1944.

Theoperationofthe apparatussetforth Fig.8willbeapparent. Withthecontactarm it engaging end contact II. the winding 21 of the motor II will be connected between the point of the inductive winding SI and the left hand output terminals 01' the phase-shifting transrormer Ilasseeninthe drawings. There willthusbeappliedtosaidwindinganalternating potential having a certain predetermined phase relation to the supply voltage as impressed upon the winding II, whereby the rotor of the motortlwilttendtorotateinaoertaindirection. with the contact arm I. he end contact of the switch 82, it will be apparent that the polarity of the alternating voltage applied to the winding 21 will be directly opposite to that which exmted when said contact arm engaged the contact 88. In other words, displacement of the'contact arm II from contact 88 to contact it will result in a complete reversal oi the phase position 0! the alternating voltage applied to the winding 21, whereby the direction of rotation of the rotor will be reversed. With the contact arm it in engagement with either of said end contacts, no voltage other than that of supply frequency will be applied to the motor winding. 1: the contact arm a be moved to the midposition, where it engages neither of end contacts 83- but does engage the contact 35, there will be no current of the supply frequency flowing in the winding fl, but said winding, on the other hand. will receive from the frequency converter 31 a voltage which, while contributing nothing to the torque of the motor, will provide a braking effect as hereinbefore set forth, whereby the motor if running, will quickly be brought to rest. There has thus been provided a reversible control embodying the principles of the invention.

In Fig. 4 is shown an application of the principle of the invention to the automatic balancing of an A.-C. bridge network utilized in the measurement of temperature. A temperature-sensitive bulb it having a predetermined electrical resistance value at a given temperature and formed of material having a known temperature coeillcient of electrical resistivity, is exposed to a temperature to be measured and is connected with other resistance units in a circuit comprising a Wheatstone bridge network. Said network includes, in addition to the variable resistance bulb II, at least three other arms, which, taken in order around a closed loop, may be enumerated as follows: a resistance unit ll having a fixed value preferably equal to the nominal resistance of the bulb in at said predetermined reference temperature, an arm comprising a slide wire portion I! of resistance value approximating the change in value of the bulb in over the operating range of temperature and a fixed portion 43 equal in value to the unit ll, together with a standardizing arm it also equal to the arm ii. The point of junction of the bulb ill and the arm II, and the point of junction of the arms 43 and 44 are connected to the output terminals of an isolating transformer I! having suitable secondary voltage rating.

Electrically engaging the slide wire 42 is a movable contact 16 adapted to be translated along said slide wire by means of a lead screw 41, and carrying an index or pointer 48, which, in cooperation with a juxtaposed graduated scale 49, provides an indication of the translated position of said contact. The contact 46 is connected by meam of a flexible conductor or equivalent 5|) to nected to the pointo! junction of the bulb l and the arm I. There is thus constituted a bridge circuit in which, upon the application of a suitable alternating voltage from the terminals of the isolating transformer l5, and by proper proportioning oi the respective arms, there will be applied to the amplifier SI an alternating potential which may be varied in intensity and phase position by changes in the relative values of said arms, and which, by suitable positioning of the movable contact #8 with respect to the slide wire II, may be reduced to zero, corresponding to a balanced condition in the bridge network; whereupon the reading of the index or pointer 48 with respect to the scale It becomes a measure of the temperature to which the bulb In is exposed. It is of course understood that, while the bridge network has been described as having iis arms nominally of equal value, the desired proportlonalitles can be obtained with other relative values, all of which is well-known in the art of electric measurement in general and electrical thermometry in particular.

A motor 55 similar in all respect to those shown in the preceding drawings, has wound upon its stator structure, two mutually displaced primary windings i and 51, with a rotor member 58 having thereon a short-circuited secondary winding. The rotor 58 is operatively connected to the lead screw 1, whereby to translate the movable contact 48 along the slide wire 42 in a direction dependent upon the direction of rota tion of said rotor. The primary winding 56 is continuously energized from an alternating current source 59 of constant frequency and voltage, from which source also is energized the primary winding of of the transformer 45.

To the output terminals of the amplifier Si is connected the primary winding of a transformer 60, having two secondary windings 6i and 62, from which may be derived electrically isolated alternating voltages of similar phase position, corresponding to that of the output of said ampliher. The secondary winding ii is connected to the terminals of the motor winding 51 in series with the secondary winding 53 of a transformer Bl, said last-named winding having an impedance value so low as not materially to obstruct the flow of current of normal supply frequency between the secondary winding GI and the motor winding 51.

Neglecting for the moment the function of the secondary winding 82 and that of the transformer 64 (presently to be explained) the apparatus as thus far set forth constitutes a selfbalancing alternating-current temperature-sensitive Wheatstone bridge adapted to continuous measurement of the temperature to which the bulb in is exposed. Upon the bridge network developing a condition of unbalance, the alternating potential due to said condition will be applied to the input terminals of the amplifier 5|, thereby causing the transformer 60 to be energized and to develop across the output terminals of each of its secondary windings an E. M. F. dependent in intensity and in phase position upon the magnitude and sense of unbalance of the bridge network. The output E. M. F. of the secondary 6 i being applied to the primary winding 51 of the motor 55, will cause said winding to produce an alternating flux which, combined with that set. up by the continuously energized winding 56, will produce a rotating held and cause the rotor 58 to 8 revoliewithaspeedandmadirectimdependent upontheunbalcnceotthebridaendwork, thus tendingtopoaitionthemblecontactll along theslidewireltinasensetorestorethebalance oithebridgaandprovidcontbesoalel! anindication of the pointer ll five oi the temperature under mt. The shift of phase of output voltage of the transformer tlLcorrespondingtoaclnnaeinthesenseofunbalanceoithcbridgenetworkwillbelmelectrical degrees, on to a complete reversal; and, should not the angular relation of the corresponding vectors to that representing the potential continuously applied to the winding 56 besuchastogivesuiinbletorqueto the motor, optimum conditions may be attained either by introducing a 1 transformer (shown dotted in Fig.4) to the unit liinFlgs. 1and2,orbyincorporaflngin the design and of the amplifier ll, by means well known in the art of electrical control, equivalent elemenm for eiiecting the desired permanent relative phase displacement between the output and input potentials of the amplifier.

The auxiliary apparatus, whereby there is applied to the motor 55 a suitably modulated retarding influence to neutralize the effects 01' inertia in the moving parts of the mechanism, is described as follows: Connected in the circuit or the secondary winding 52 on the transformer 50 is a diode or rectifying tube 66 having an anode and a cathode, the latter preferably grounded. In series with the diode i6 and the transformer winding 62 is a resistor 61, one end of which is connected to the ground point of the circuit, 1. e. the cathode of the tube 66. Thus, at all times of unbalance of the bridge network, with a consequent alternating voltage from the secondary winding ii tending to cause operation of the motor 55, a similar alternating voltage from the secondary winding 62 will be impressed upon the diode 66, whereby a rectifier current will flow through said diode and the resistor 61, causing a unidirectional potential gradient to be established across the same. A suitable capacitor 68, connected across the terminals of the resistor 61, will introduce a filtering eiiect, tendin to eliminate pulsations from said potential; and, if desired, a lull-wave rectifying unit may be substituted in the circuit, whereby to produce a direct current flow and a corresponding potential, variable in magnitude with the degree of unbalance of the bridge network.

A transformer Ill, having a primary and a secondary winding, is adapted to be energized from a supply II of alternating current, having a irequency diifering materially from that of the main supply source 59. A triode 12, having a cathode, rendered emissive by a heating source not shown, a grid, and a plate or anode, has said cathode connected to a grounded conductor 13 in series with a suitable biasing resistor H. A battery 15, or other suitable source 01' direct current has its negative terminal connected to the grounded conductor 18 and its positive terminal in series with a suitable resistor N, to a conductor 11 attached to the plate of the triode 12. A by-pass capacitor I! is connected in parallel with the resistor I4, between the cathode of the tube 12 and the grounded conductor 13. The grid of the triode I2 is connected through a current limiting resistor I! to a selected tap on the resistor 61, whereby there will be applied upon said grid a potential to the cathode corresponding to the value of current flowing in the resistor 01. The grid of the tube 12 is connected also to one terminal of the secondary winding of transformer it through a blocking capacitor til, which prevents the how of direct current in the transformer winding; and the other terminal of said winding is connected to the grounded conductor l3. A second triode 82, having a cathode, rendered emissive by heating means not shown, a grid and a plate, has said cathode connected to the grounded conductor is in series with 'a suitable biasing resistor 83, having in parallel therewith a bypassing capacitor 54. The positive terminal of the battery is connected through the primary winding of the transformer 64 to the plate of the triode B2. The'grid of the triode 82 is connected through a capacitor 85 to the conductor l1, and also through a resistor 85 to the grounded conductor II. a

The operation of the motor 55 in a sense to restore a balanced condition to the bridge network has already been explained; and it has also been shown that concomitantly with the operation of the motor there will appear across the resistor 81 a unidirectional potential having a ma itude varying with that of the output of the amplifier II, and, therefore, with the intensity of the motor operating influence. Because, however, the diode 68 in its rectifying function disregards phase position of the alternating current to which it is subjected, the unidirectional potential gradient along the resistor 51, while varying in magnitude with operation of the motor 55, will be independent of the direction of said rotation and can have only one polarity; and said polarity, with connections as indicated, will be such that the tapped point 01 the resistor 51 will be negative to ground, thus applying a corresponding grid potential to the triode l2. Alternating voltage derived from the secondary winding of the transformer Ill and applied between the grid and the cathode of the tube 12 in series with the capacitor 80, will affect the flow of current from the battery 15 through the plate circuit, whereby the output current will be characterized by an alternating component of frequency corresponding to that of the source II, and as such will flow from the plate of said tube through the conductor 11, the resistor 15, and the battery 15, as well as through the resistor 14 and the capacitor 18 in parallel. The tap on the resistor 51 being suitably selected, the negative bias derived from the resistor 68 may be made such as more or less to block the flow of said current in the output circuit of the tube 12, whereby when no voltage exists across the terminals of the secondary winding 62 the grid bias of the tube 12 will be a fixed value, and as the output voltage of the winding 52 increases, said grid bias will correspondingly increase, until a value is reached at which the magnitude of the output current of the tube 12 becomes zero.

The output current of the tube I2, flowing in the resistor 18, will establish across the same a corresponding potential drop of which the alternating component will be applied through the blocking capacitor 85 to the grid of the triode 82, whereby to cause to flow in the plate circuit of the same a fluctuating current of corresponding frequency and magnitude. There will thus be set up in the secondary winding 63 of said transformer an alternating voltage which will combine with the voltage of the secondary winding Bi of the transformer 50 to produce current in the primary winding 51 of the motor 55. Be-

cause of the bias of the tube 12 varying, as previously pointed out, with the magnitude of output of the transformer 50, the output of the transformer Bl will tend to vary in a sense opposite to that of the transformer 50, withthe result that, as a condition of balance in the bridge network is approached, and the driving force on the motor 55 progressively lessened, the component of current from the transformer 64 having the frequency of the source I i is correspondingly increased. Since said component, being of a frequency other than that of the flux produced by the winding 56, and therefore contributing nothing to the torque, acts only to provide a braking or retarding influence on the rotor, as hereinbefore set forth, it follows that there has been effected the desirable result of applying to a servomotor a retarding eflect which shall be a minimum or zero at the time the motor is required to. operate at its greatest speed in either direction, increasing as the driving force on the motor is increased, and attaining a maximum value as the motor torque falls to zero. The rate of variation of bias of the triode 12 with respect to the output voltage of winding 52 can be controlled not only by selection of the characteristics of said triode, but by suitably selecting and coordinating the values of the resistors 61, .19 and II. If a gradual reduction in the braking or retarding component is desired there may be used for the triode 12 a tube having a "variable mu" characteristic, well known in the art of electronics.

In copending application Serial No. 521,236, filed by W. H. Bussey Feb. 5,1944, now Patent No. 2,444,726. there is set forth a method and apparatus whereby the non-linear characteristics of certain ferromagnetic systems may be utilized for the purpose of amplifying small unidirectional potentials such as characterize the unbalance of direct current bridge and potentiometer networks, and obtaining therefrom representative alternating currents of a nature which may be applied to the operation of a servomotor for restoring a condition of balance and providing a measure of the magnitude of the unbalance influence. It is also shown that where such a system is given suitable alternating excitation there will appear in the output various alternating components including one having the fundamental frequency and varying in amplitude and in phase position with the intensity and polarity of the unbalance potential."

In Fi 5 is shown diagrammatically an electromagnetic unit 88 of the type set forth in said Bussey application and comprising a modulator element 59 including a ferromagnetic core structure having a middle leg 5|! and outside legs 9| and 92, said legs being provided with suitable windings comprising a coil 93 on the leg 90 and mutually identical coils 94 and 85 on the legs 9| and 92 respectively. The coils 94 and 95 are intel-connected with their relative polarity such that current flowing through them in series will tend to magnetize the core portions 9i and 92 in opposite sense, i. e. in a sense to make the upper end of one of said legs, as seen in the drawing, of the same magnetic polarity as the lower end of the other of said legs. The free ends of the windin composed of the coils 9i and 95 are connected through an inductive choke 98 to terminals 99. to which may be applied a D.-C. potential to be investigated, and also through a capacitor Hill, which effectually bars the flow of direct current, to an amplifier iili tuned to characteristics account 11 presentlytobesetformandthencetodemodulating unit It! having output terminals I03 and I lit.

Thecoiillissuppliedwith exciting current from two interconnected A.-C. sources It and 01, producing alternating currents of high and low frequencies respectively. these currents being superimposedinthewindinglltocombinetheir magnetizing influences on the magnetic system. with the fundamental or lower frequencyoftheorderoftocyclespersecondit has been found that eilective operation of the apparatus is obtained when the higher frequency lies in the "audio" range of which a value of l650cyelesperseoondmaybetakenasane2- ample. The tuning of the amplifier III is made such that it will pass a band of frequencies ranging from the sum to the difference of those derived from the sources It and II.

It may be shown that when the two frequency components (designated as 11 and I: respectively) are superimposed in an elic system as hereinbefore described. and a unidirectional unbalancing influence introduced, there will appear in the output voltage components having a number of different frequencies, among which will be those having values of (fz+i1) and 15-11). If there be added to these a third component having a frequency is, the resultant voltage may be considered as a modulated wave, from which the component having the frequency 11 mayberecoveredbytheuseofanyoneofa number of suitable demodulating networks wellknown in commimication practice. By virtue of the frequency component fa, acting as a carrier wave, and having a much higher frequency than the fundamental h, it follows that amplification canbeeil'ectedtoanydesireddegreewithcor respondingly greater case than could be done were the high-frequency component not present. Should the volta e applied to the amplifier IIII not contain a sumciently high proportion of the high frequency component to provide the required modulation characteristics, such may be artillcially" introduced by means of a circuit it! including a rheostat "it, deriving from the high frequencysouree 96 a component having a frequency h and superimposing said component upon the output of the induction apparatus.

While it has been thought desirable to eliminate involved mathematical analyses from the present discussion, the method by which ap ropria e computations may be carried out are wellknown in the art. and upon these may e established a number of outstanding facts: the urincipal of which is that the component of t e output voltage having the frequency II will varv in phase position with the direction of the 11-0. nbalancing influence. and in amplitude with the quantitative value of said influence While the high (audio) frequency output I: will be substantiall eliminated by the demodulator I02, I flnd it possible to obtain in the demodulator out ut a second harmonic component of the low frequency fl ii. e.. a component having the frequency 2h) which second harmonic component varies in amplitude inversely as the amplitude of the low or fundamental frequency (m voltage in the demodulator output. This is obtained when the amplitude of the high frequency (is) and the low frequency ([1) input currents are approximately equal. Thus there will remain in the demodulator output a double frequency component, which, when applied to one of the primary windings of a two-phase induction motor,

12 will. as hereinbefore set forth. contribute nothingtothedrivingtorqucbutwllltendtoinhibit rotation. and, when the fundamental component is eliminated will bring the rotor qui kly to rest. In Fig. 6 is shown the application. according to the principle of the invention, of an'induction modulator of the type shown mm. 5 to operation of a servomotor in a mechanism for automatically a potentiometer adapted to thepurposesofpyrometrybyprovidlngacon tinuous measure of the E. M. 1'. developed in a thermocouple exposed to a temperature to measured A slide-wire Ill carrying a constant current derived from a battery IIII, and adjusted to a suitable predetermined value by means of a rheostat I", is adapted to betraversed by',,a sliding contact IlIi positioned by means of a lead screw III which may be driven in either direction by a reversible electric motor III. An index or pointer III in cooperation with a graduated scalellt provides a measure of the positlon of the sliding contact Ill ,with respect to the slide-wire llil. A thermocouple iii is exposedtothetemperaturetobemeasured.and one end of said couple is connected by means of a flexible lead lit to the sliding contact Illi. Do the .lefthand end of the slide-wire Iiil. as seen in the drawing. is connected a conductor Ill, and the free end of the thermocouple II! connected to a conductor lit.

An electromagnetic unit Ill. similar in all respects to the hereinbefore described unit 8! shown in Fig. 5, includes a modulating element In (corresponding to element 80) and is provided with a source I II of low frequency current and a source I22 of high frequency current, whereby there may be derived the dual excitation fully described in connection with the apparatus shown in Fig. 5. The secondary output circuit of the element I20 is provided with an amplifier I28 and a demodulator I24; and the 11-0. input terminals of the unit I20 are connected in series with a suitable inductive choke III to the conductors Iii and III, and thereby included in the potentiometer circuit.

The motor II! is preferably of the two-phase type, having two mutually displaced primary windings I26 and I21. The winding I26 is continuously excited from the source I2I, whereby there is provided an alternating ileld in the motor. If desired, there may be placed in series with the winding I26 and the source III a phaseshifting device I28, whereby the phase position of the ileld developed by the winding I26 may be most advantageously located. The winding III is connected to the output terminals of the demodulator I24, and will thus, as hereinbeiore set forth, receive an alternating current having a component of fundamental frequency and varying in magnitude and phase position with changes in the E. M. F. derived from the conductors Ill-Iii.

In so far as operation on fundamental frequency is concerned, the performance of the self-balancing instrument is identical with that set forth in the above mentioned Bussey application. As the temperature to which the thermocouple H5 is exposed varies. the balance between the thermoelectromotive force derived therefrom and that corresponding to the position of the sliding contact IIII on the slide-wire IIII will be disturbed; and the differential E. M. l"., appearing between the conductors II'I--II8 will be applied to the windings of the modulating element I20, and wfll cause an alternating E. M. F. having a component corresponding in frequency to that of the source I21 and dependent in magnitude and phase position upon the magnitude and polarity of the unbalanced unidirectional E. M. F., to appear at the output terminals of the demodulator I24, and to be applied to the primary winding I21 of the motor 2. The motor H2 will operate in a direction and with a velocity depending upon the phase position and intensity of the current in its winding I27, whereby said motor may be caused to position the sliding contact I along the slide wire I01 in a sense to reduce the unbalance; and the position of said contact, as exhibited by the pointer I23 on the scale.l2l will be a measure of the temperature to which the thermocouple III is exposed.

As pointed out hereinbefore, I find that when the amplitude of the high and low frequency input currents from the sources l2! and I22 are approximately equal, there is obtained in the output of the demodulator l2 a pronounced alternating component having a frequency double that of the fundamental f1, and furthermore having an amplitude which varies inversely with that of the fundamental frequency component in said demodulator output. The amplitude of said double frequency component is moreover independent of the phase position of said fundamental in relation to the source of supply. The current supplied to the winding I21 of the motor 2 contains therefore a component. which will apply to the rotor of the motor a retarding influence tending to be at a minimum when the deviation of the thermocouple voltage from the slide-wire potential is greatest, and to rise to a maximum as a condition of balance is approached.

The terms and expressions which I have employed are used as terms of description and not of limitation, and I have no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but recognize that various modifications are possible within the scope of the invention claimed.

I claim:

1. Apparatus for controlling an electric motor of the type having two dephased primary windings cooperative with a rotor, said apparatus comprising an induction balance, means for producing alternating magnetic fields linked with the secondary windings of said balance by applying concurrently to the primary winding alternating E. M. F.s of substantially different frequencies but approximately equal amplitudes, means for superimposing upon portions of the magnetic circuits of said induction balance windings magnetic forces derived from a undirectional current of magnitude corresponding to that of a variable condition and aflecting said magnetic circuit portions in opposite directions, means for deriving from the output voltages of said secondary windings a voltage having the frequency of the lower of said input frequencies and a voltage having a frequency which is a multiple thereof and which varies in amplitude inversely with that of said derived lower frequency voltage, means for applying both said derived voltages to one of the windings of said motor, and means for applying to the other winding of said motor a voltage having said lower frequency.

2. Apparatus for controlling an electric motor of the type having two dephased primary windings cooperative with a rotor, said apparatus comprising an alternating current Wheatstone bridge, means responsive to unbalance of said bridge for developing an E. M. F. dependent in amplitude and phase position upon the magnitude and sense of unbalance of said bridge, means for applying said E. M. F. to one of the windings of said motor, means for applying to the other winding of said motor an E. M. F. of the same frequency as the first mentioned E. M. F., means for developing a unidirectional potential having a magnitude varying with that of said first mentioned E. M. F., and electronic means responsive to said unidirectional potential for applying to the first mentioned winding of said motor an E. M. F. of frequency different from that in the other motor winding and providing a retarding force which increases as the motor driving force 1} decreases.

ABRAHAM WALTER JACOBSON.

REFERENCES crrsn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 20,568 Bedford Dec. 7, 1937 871,513 Lohr Nov. 19, 1907 1,450,339 Smith et al.' Apr. 8, 1923 1,748,078 Prantl Feb. 25, 1930 2,141,056 Watkins Dec. 20, 1938 2,172,064 Harrison Sept. 5, 1939 2,273,191 Harrison Feb. 17, 1942 2,355,537 Jones Aug. 8, 1944 2,367,868 Jones Jan. 23, 1945 2,411,357 Bertram Nov. 19, 1946 Certificate of Correction Patent No. 2,500,314 4 March 14, 1950 ABRAHAM WALTER JACOBSON It is herehy certified that error appears in the printed specific'ation of the above numbered potent requiring correction as follows: t 7

Column 14, line 3, for undirectional reed unidirectional; and that the aid Inttem Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Oflioe.

Signed and sealed this 11th day of July, A. D. 1950.

THOMAS F. MURPHY,

Areiatant Oommim'om of PM. 

